Re: [CAD_CAM_EDM_DRO] Re: capacitor

mariss92705 wrote:

> Leslie,
>
> That is a good "worst-case" approximation but it is overly
> pessimistic. It neglects to take into account the rate of energy
> return.
>
> A burning stick of dynamite releases the same amount of energy as one
> set off by a blasting cap; the difference is the span of time that
> energy is delivered.
>
> It's the same here. Friction and electrical losses in the motor and
> drive absorb the returned energy unless the rate of return is so
> large (very rapid deceleration of a large mass from a high speed)
> that it results in a net negative power supply current.
>
> This phenomena is of more concern with step motor because their "back
> EMF" can be many times the supply voltage.

What? How is this possible? The back EMF, strictly, can't be any
greater than the voltage required to MOVE the motor at that speed.
This doesn't count inductive effects, so if you short the winding while
it is moving, and then remove the short, high voltages could be
developed.

> The DC servodrive does not
> exhibit this effect at all. Deceleration at any rate or load
> generates no returned power, ever.

Now I KNOW you stayed up too late last night! This, too, is false.
How can the energy inherent in motion (inertia) be dissipated without
going anywhere? Most certainly, energy CAN be reterned to the power
supply. Your drives may have topology that prevents this, mine
CLEARLY return energy to the supply, as the voltage on the cap bank
rises when the motors are decelerated.

If anyone doubts this, connect a light bulb to a servo motor and turn the
shaft. The bulb (if appropriate voltage and current) will light up, and get
brighter the faster you turn the motor. How is this different from decelerating
a machine tool?

If you mean that the back EMF of the motor must always be less than the DC
power supply, then you are ignoring the inductive effects of the PWM
techniques. Linear servo amps are not generally able to produce regenerative
braking, and disspate the full energy as heat in their power transistors. But,
switching-type servo amps definitely can send the energy back to the
power source.

Jon